Was the 'ring gap" set and checked. It only takes a couple of minutes for "too little" gap to stop the engine when the ring gap goes away due to cylinder heat and that will bring the engine to a stop. Once cooled they can have enough clearance to turn freely. They can also make a scraping sound when tight.
Even the "proper pre-gapped rings" need to be checked.
The slow stop of the engine and being free when cooled lead me to this "possibility". Hopefully I'm wrong.
Did the engine turn freely before starting?
Dave Neptune